That is a very poor circuit, IMHO. There is no way a 12AX7 concertina splitter should be driving parallel PP EL34s in UL mode. Putting a 12AU7 common cathode amp stage on the front is a bad idea because of the tube's poor linearity and the splitter's bias circuit is wrong too.

The only way to get sensible performance with parallel pairs with low thd is either to use cathode follower drivers or an intermediate balanced push-pull voltage driver stage as per Williamson. I favour the latter as it cushions the split load.. I fail to understand why the authors of the kit haven't attempted this avenue.

Academically, readers can come to a swift sum-up by browsing Morgan Jones 3rd ed, classic amplifiers p. 412 onwards "the inherent driver defects of the Mullard 5-20". That's only with a conventional p-p pair not paralleled.
No hiding facts here.

The ECC83 split load as given will give a maximum peak signal output of about 40V before it runs out of steam (quickly hooked up) - and that is at quite visible 2nd harmonic distortion on the scope. (A 47K plate load in parallel with only 110K next grid resistors is looking for distortion.) Then grid-leak bias only works well when fed with a low impedance source. A feeding ECC83 is definitely not, apart from the grid leak resistor being too low - at least 10meg should have been there. Or it could have been direct coupled, although hardly successfull with an ECC83. I have said in the past that I do not see the ECC83 as a good choice for any kind of duty in a power amplifier. A pentode input to low mu triode phase-splitter would run circles round the above (ECF80, ECF82, 7199 etc. - and no comment from the "pentode-is-noisier" clan. That is in theory; certainly not audible in this kind of application.

I thought this kind of circuit has already been thrashed out to such an extent over 6 decades that should no longer appear compromised, . My apologies, but if this is the best Velleman can come up with, they have lost me.

The ECC83 (EF86, triode of the ECL86, ECLL80) are tubes, that can work with grid leak bias.
So this concertina phase inverter with grid leak bias we also meet in other amplifiers: Revox G36, Telewatt VS55, Grundig NF10.

The datasheet for the ECL86 (triode) gives an output voltage of 9 Volt RMS with THD 0,4%.